We consider a geometrically thin , Keplerian disk in the orbital plane of a binary black hole ( BHBH ) consisting of a spinning primary and low-mass secondary ( mass ratio q \lesssim 1 ) .
To account for the principle effects of general relativity ( GR ) , we propose a modification of the standard Newtonian evolution equation for the ( orbit-averaged ) time-varying disk surface density .
In our modified equation the viscous torque in the disk is treated in full GR , while the tidal torque is handled in the Newtonian limit .
Our GR-hybrid treatment is reasonable because the tidal torque is concentrated near the orbital radius of the secondary and is most important prior to binary-disk decoupling , when the orbital separation is large and resides in the weak-field regime .
The tidal torque on the disk diminishes during late merger and vanishes altogether following merger .
By contrast , the viscous torque drives the flow into the strong-field region and onto the primary during all epochs .
Following binary coalescence , the viscous torque alone governs the time-dependent accretion onto the remnant , as well as the temporal behavior , strength and spectrum of the aftermath electromagnetic radiation from the disk .
We solve our GR-hybrid equation for a representative BHBH-disk system , identify several observable EM signatures of the merger , and compare results obtained for the gas and EM radiation with those found with the Newtonian prescription .